This invention relates to a process for the removal of hydrogen sulfide from gaseous mixtures by scrubbing with an aqueous alkaline scrubbing solution containing an oxidizing agent. In particular, the hydrogen sulfide is absorbed from the scrubbing solution and oxidized principally to elementary sulfur; the resultant scrubbing solution containing the oxidizing agent in a reduced valent state is regenerated in a downstream oxidizer by treatment with an oxygen-containing gas; and the resultant scrubbing stream containing reoxidized oxidizing agent, after the removal of elementary sulfur, is then recycled to the scrubbing column. (The sulfur may alternatively be removed prior to the oxidizing step.)
A process of this type is known, for example, from Brennstoffchemie (Fuel Chemistry), Volume 50, 1969, No. 4, pages T24 and T25. According to the process described in this reference, the hydrogen sulfide is initially bound by the alkali as a hydrogen sulfide and then reacts with the oxidizing agent, with liberation of sulfur and reduction of the oxidizing agent. The oxidizing agent is preferably a salt of anthraquinonedisulfonic acid and/or five-valent vanadium; and other oxidizing agents can also be used, such as, for example, salts of iron, copper, chromium, nickel, cobalt or manganese. In order to reactivate the solution, air is blown through the oxidizer. The reduced oxidizing agent is then oxidized, and simultaneously, the sulfur floats to the surface of the solution and collects there as a scum. The supernatant sulfur is drawn off, filtered and then autoclaved.
The oxidation of the hydrogen sulfide, e.g., NaHS, occurs extremely rapidly, so that hardly any hydrogen sulfide reaches the oxidizer; thus the main condition for the formation of thiosulfate, which is observed in other processes, is almost eliminated. Nevertheless, even in this process, so much thiosulfate as well as sulfate are still formed that, due to their great solubility in the scrubbing solution, a buildup of thiosulfate in particular occurs which interferes with the efficiency of the scrubbing solution.
It has already been recognized by Beavon in U.S. Pat. No. 3,642,448 that the cause of the formation of thiosulfate and sulfate can be attributed to the presence of oxygen in the scrubbing solution. This depends on the fact that, after the oxidation of the scrubbing solution, dissolved oxygen reaches the scrubbing column and therein can form sulfate or thiosulfate with the hydrogen sulfide. To solve the problem, Beavon teaches the incorporation of an additional stage comprising stripping the oxygen from the absorption solution prior to contacting the latter with H.sub.2 S. See also Fleck U.S. Pat. No. 4,125,597 for another mass transfer technique to remove oxygen. Conversely, if the concentration of oxidizing agent, e.g., five-valent vanadium, in the scrubbing step does not satisfy the stoichiometric quantity required for the content of hydrogen sulfide, non-oxidized hydrogen sulfide reaches the downstream oxidizer therein to form thiosulfate and sulfate by-products.